Process for the hot drawing of polybenzimidazole fibrous materials wherein a heated draw surface is employed

ABSTRACT

POLYBENZIMIDAZOLE FIBROUS MATERIALS ARE KNOWN TO POSSESS AN INHERENT TENDENCY TO ABSORB MOISTURE FROM THE ATMOSPHERE. AN IMPROVED PROCESS IS PROVIDED FOR DRAWING A POLYBENZIMIDAZOLE FIBROUS MATERIAL WHILE IN CONTACT WITH A HEATED DRAW SURFACE (E.G., ONE OR MORE HOT SHOES). IMMEDIATELY PRIOR TO CONTACT WITH THE HEATED DRAW SURFACE THE POLYBENZIMIDAZOLE FIBROUS MATERIAL WHILE IN ASSOCIATION WITH A MINOR QUANTITY OF MOISTURE (E.G., ABOUT 0.5 TO 2 PERCENT BY WEIGHT) IS CONTACTED WITH AN AUXILIARY HEATED SURFACE IN THE ABSENCE OF ELONGATION AND BROUGHT TO A TEMPERATURE BELOW THE GLASS TRANSITION TEMPERATURE OF THE POLYBENZIMIDAZOLE WITH THE SIMULTANEOUS REMOVAL OF MOISTURE. A DRAW TECHNIQUE OF SATISFACTORY STABILITY IS PROVIDED AND A SUPERIOR DRAWN FIBROUS PRODUCT IS PRODUCED.

United States Patent O i 3,836,621 PROCESS FOR THE HOT DRAWING OF POLY- BENZIMIDAZOLE FIBROUS MATERIALS WHEREIN A HEATED DRAW SURFACE IS EMPLOYED Arthur E. Prince, Jr., Charlotte, N.C., assignor to Celanese Corporation, New York, N.Y.

No Drawing. Continuation-in-part of application Ser. No. 239,898, Mar. 31,1972. This application Nov. 3, 1972, Ser. No. 303,456

Int. Cl. D01d 5/12 US. Cl. 264-210 F 8 Claims ABSTRACT OF THE DISCLOSURE Polybenzimidazole fibrous materials are known to possess an inherent tendency to absorb moisture from the atmosphere. An improved process is provided for drawing a polybenzimidazole fibrous material while in contact with a heated draw surface (e.g., one or more hot shoes). Immediately prior to contact with the heated draw surface the polybenzimidazole fibrous material while in association with a minor quantity of moisture (e.g., about 0.5 to 2 percent by Weight) is contacted with an auxiliary heated surface in the absence of elongation and brought to a temperature below the. glass transition temperature of the polybenzimidazole with the simultaneous removal of moisture. A draw technique of satisfactory stability is provided and a superior drawn fibrous product is produced.

CROSS REFERENCE TO RELATED APPLICATION This is a continuation-in-part of my commonly assigned U.S. Ser. No. 239,898, filed Mar. 31, 1972, and entitled Drying and Drawing Process for Polybenzimidazole Continuous Filamentary Materials.

BACKGROUND OF THE INVENTION The manufacture of a continuous length of polybenzimidazole fibrous material is wellknown in the art. A solution of polybenzimidazole in an appropriate solvent is formed into acontinuous filamentary material, and the resulting filamentary material is washed, dried, and hot drawn. g

The hot drawing of a continuous length of polybenzimidazole fibrous material is a known technique for increasing its tensile properties, and particularly its tenacity. Heretofore continuous lengths of polybenzimidazole fibrous materials have been hot drawn by applying a longitudinal tension thereto while being heated to a temperature at or above the glass transition temperature of' the polybenzimidazole. Commonly, polybenzimidazole fibrous materials exhibit a glass transition temperature within the range of about 250 to 450 C. Such hot drawing has been accomplished while the polybenzimidazole fibrous materials are (l) incontact with a hot surface (e.g., one or more hot shoes) or (2) while passing through a radiantly heated drawing zone in which the fibrous materials are suspended.

Polybenzimidazoles unlike most other synthetic and natural fibers have a relatively high natural moisture regain level, i.e., about 13 percent at ambient conditions (i.e., at a temperature of about 25 C. and a relative humidity ofabout 65 percent), and .have an inherent tendency rapidly .to absorb moisture from the atmosphere at ambient conditions. Most polymeric materials have a relatively -;low natural moisture regain level. Polyethylene terepl'ithalate, for example, has a natural moisture regain level of, about 0.5 percent. Cotton has a natural moisture regain level ofabout percent. It is relatively easy to femove moisture from a polybenzimidazole fibrous material down to about the natural moisture regain but removal of moisture below that level has proven to be difficult.

It has heretofore been recognized that it is desirable to remove moisture from a polybenzimidazole fibrous material to at least some degree prior to hot drawing if optimum tensile properties are to be achieved. See, for instance, U.S. Ser. No. 239,898, filed Mar. 31, 1972, which is directed primarily to an in-line continuous process for drying and drawing a continuous length of a polybenzimidazole fibrous material. The non-destructive removal of minor quantities of moisture while in association with the polybenzimidazole fibrous material (i.e., water in a concentration of about 0.5 to 2 percent by weight) has proven to be difiicult and often highly time consuming since such trace amounts of water tenaciously adhere to the fibrous material. Also, some polybenzimidazole fiber samples appear unexplainably to retain trace amounts of moisture more tenaciously than others. The presence of a minor amount of water in association with the polybenzimidazole fibrous material generally has proven to be particularly disadvantageous when heat is imparted to the same during drawing by contact with a hot draw surface (e.g., a hot shoe) as opposed to a ra diant heat application. The conduction heating during drawing of the polybenzimidazole fibrous material containing a minor quantity of water has accordingly proven to have a deleterious influence on tensile properties.

It is an object of the present invention to provide an improved process of satisfactory stability for the hot drawing of a polybenzimidazole fibrous material wherein a heated draw surface (e.g., one or more hot shoes) is employed.

It is an object of the present invention to provide an improved process for the hot drawing of a polybenzimidazole fibrous material employing a heated draw surface wherein difficulties heretofore encountered because of the presence of a minor quantity of moisture in association with the polybenzimidazole fibrous material are effectively eliminated.

It is an object of the present invention to provide an improved process for the hot drawing of a polybenzimidazole fibrous material employing a heated draw surface wherein a minor quantity of moisture is rapidly and efficiently removed from the same in a substantially nondeleterious manner immediately prior to such hot drawing.

It is another object of the present invention to provide an improved process for the hot drawing of a polybenzimidazole fibrous material employing a heated draw surface which is capable of yielding a fibrous product exhibiting enhanced tensile properties.

It is a further object of the present invention to provide an improved process for the hot drawing of a polybenzimidazole fibrous material employing a heated draw surface which is capable of yielding a fibrous product which is substantially free of opaque streaks.

These and other objects, as well as the scope, nature, and utilization of the process will be apparent to those skilled in the art from the following detailed description and appended claims.

SUMMARY OF THE INVENTION It has been found that in a process for the hot drawing of a continuous length of polybenzimidazole fibrous mate: rial which is in association with a minor quantity of moisture (e.g., about 0.5 to 2 percent by weight) through the application of a longitudinal tension thereto While in contact with at least one heated draw surface from which sufficient heat is imparted to fibrous material to heat the same to a temperature which exceeds the glass transition temperature of the polybenzimidazole; that improved results are achieved by preliminarily contacting the continuous length of polybenzimidazole fibrous material with at least one auxiliary heated surface immediately prior to contact with the heated draw surface in the absence of elongation whereby sufiicient heat is imparted to the fibrous material to heat the same to a temperature within the range of about 200 C. up to below the glass transition temperature of the polybenzimidazole with the simultaneous removal of moisture present in association therewith.

DESCRIPTION OF PREFERRED EMBODIMENTS The Starting Polymer Polybenzimidazoles are a known class of heterocyclic polymers. Typical polymers of this class and their preparation are more fully described in U.S. Pat. No. 2,895,948, US. Reissue Pat. No. 26,065, and in the Journal of Polymer Science, Vol. 50, pages 511-539 (1961) which are herein incorporated by reference. The polybenzimidazoles consist essentially of recurring units of the following Formulas I and II.

Formula I is:

wherein R is a tetravalent aromatic nucleus, preferably symmetrically substituted, with the nitrogen atoms forming the benzimidazole rings being paired upon adjacent carbon atoms, i.e., ortho carbon atoms, of the aromatic nucleus, and R is a member of the class consisting of (1) an aromatic ring, ('2) an alkylene group (preferably those having 4 to 8 carbon atoms), and (3) a heterocyclic ring from the class consisting of (a) pyridine, (b) pyrazine, (c) furan, (d) quinoline, (e) thiophene, (f) pyran.

Formula II is:

wherein Z is an aromatic nucleus having the nitrogen atoms forming the benzimidazole ring paired upon adjacent carbon atoms of the aromatic nucleus.

Preferably, aromatic polybenzimidazoles are selected, e.g., polymers consisting essentially of the recurring units of Formulas I and H wherein R is an aromatic ring or a heterocyclic ring.

As set forth in US. Reissue Pat. No. 26,065, the aromatic polybenzimidazoles having the recurring units of Formula II may be prepared by self-condensing a trifunctional aromatic compound containing only a single set of ortho disposed diamino substituents and an aromatic, preferably phenyl, carboxylate ester substituent. Exemplary of polymers of this type is poIy-2,5(6)-benzi midazole prepared by the autocondensation of phenyl-3,4- diaminobenzoate.

As also set forth in the above-mentioned patent, the aromatic polybenzimidazoles having the recurring units of Formula I may be prepared by condensing an aromatic tetraamine compound containing a pair of orthodiamino substituents on the aromatic nucleus with a dicarboxyl compound selected from the class consisting of (a) the diphenyl ester of an'aromatic dicarboxylic acid, (b) the diphenyl ester of a heterocyclic dicarboxylic acid wherein the carboxyl groups are substituents upon a carbon in a ring compound selected from the class consisting of pyridine, pyrazine, furan, quinoline, thiophene and pyran and (c) an anhydride of an aromatic dicarboxylic acid.

Examples of polybenzimidazoles which have the recurring structure of Formula I are as follows:

poly-2,2 (b-phenylene -5, 5 '-bibenzimidazole; poly-2,2'-(pyridylene-3",5)-5,5'-hibenzirnidazole; poly-2,2-(furylene-2",5")-5,5'-bibenzimidazole; poly-2,'2-(naphthalene-1",6")-5,5-bibenzimidazole; poly-2,2-(biphenylene-4",4")-5,5'-bibenzimidazole; poly-2,2'-amylene-5,5'-bibenzimidazole; poly-2,2'-octamethylene-5,5-bibenzimidazole; poly- 2, 6- (m-phenylene) -diimidazo'benzene; poly-2,2-cycloheXeneyl-5,5bibenzimidazole; poly-2,2(m-phenylene)-5,5'-di(benzimidazole) ether; poly-2,2 (m-phenylene) -5,5'-di(benzimidazole) sulfide; poly-2,2"(m-phenylene)-5,5-di(benzimidazole) sulfone; poly-2,2(m-phenylene)-5,5-di(benzimidazole) methane; poly-2,2"' (m-phenylene) -5 ',5" 7 di (benzimidazole) propane-2,2; and poly-2',2(mphenylene)-5,5" di(benzimidazole) ethylene-l,2

where the double bonds of the ethylene groups are intact in the final polymer.

The preferred polybenzimidazole for use in the present process is one prepared from poly-2,2'-(m-phenylene)-. 5,5-bibenzimidazole, the recurring unit of which is:

Any polymerization process known to those skilled in the art may be employed to prepare the polybenzimidazole which may then be formed into a continuous length of fibrous material. Representative techniques for preparing the polybenzimidazole are disclosed in US. Pats. Nos. 3,509,108, 3,549,603, and 3,551,389, which are assigned to the assignee of the present invention and are herein incorporated by reference.

With respect to aromatic polybenzimidazoles, preferably equimolar quantities of the monomeric tetraamine and dicarboxyl compound are introduced into a first stage melt polymerization reaction zone and heated therein at a temperature above about 200 C., preferably atleast 250 C., and more preferably from about 270 to 300 C. The reaction is conducted in a substantially oxygen-free atmosphere, i.e., having below about 20 ppm. oxygen and preferably below about 8 ppm. oxygen, until a foamed prepolymer is formed having an inherent viscosity, expressed as deciliters per gram, of at least 0.1, and preferably from about 0.13 to 0.3, the inherent viscosity (I.V.) as used herein being determined from a solution of 0.4 grams of the polymer in ml. of 97 percent H SO at 25 C.

After the conclusion of the first stage reaction, which normally takes at least 0.5 hour and preferably 1 to' 3 hours, the foamed prepolymer iscooled and then powdered or pulverized in any convenient manner. The resulting prepolymer powder is then introduced into a second stage polymerization reaction zone wherein it is heated under substantially oxygen-free conditions, as described above, to yield a polybenzimidazole polymer product, desirably having an I.V., as measured above, of at least 0.6, e.g., 0.80 to 1.1 or more. i

The temperature employed in the second stage is at least 250 C., preferably at least 325 C., and more preferably from about 350 to 425 'C. The second stage reaction generally takes at least 0.5 hour, and preferably from about 1 to 4 hours or more. I

A particularly preferred method for preparing the polybenzimidazole is disclosed in the aforesaid US. Pat. No. 3,509,108. As disclosed therein aromatic polyben zimidazoles may be prepared by initially reacting the mono;

mer in a melt phase polymerization at a temperature above about 200 C. and a pressure above 50 p.s.i. (e.g., 300 to 600 p.s.i.) and then heating the resulting reaction product in a solid state polymerization at a temperature above about 300 C. (e.g., 350 to 500 C.) to yield the final product.

Preparation of the Continuous Length of Fibrous Material I The term continuou length of polybenzimidazole fibrous material as used herein includes monofilaments, as well as multifilament fibrous materials, such as yarn, strand, cable, tow, and the like. In preferred embodiment of the process the continuous length of polybenzimidazole fibrous material is a multifilament yarn or a multifilament tow.

As is known in the art, polybenzimidazoles are generally formed into continuous lengths of fibrous materials by solution spinning, that is, by dry or wet spinning a solution of the polymer in an appropriate solvent such as N,N-dimethylacetamide, N,N-dimethylformamide, dimethylsulfoxide or sulfuric acid (used only in wet spinning) through an opening of predetermined shape into an evaporative atmosphere for the solvent in which most of the solvent is evaporated (dry) or into a coagulation bath (wet), resulting in the polymer having the desired filamentary shape.

, The polymer solutions may be prepared in accordance with known procedures. For example, sufficient polybenzimidazole may be dissolved in the solvent to yield a final solution suitable for extrusion containing from about to 45 percent by weight of the polymer, based on the total weight of the solution, preferably from about 20 to 30 percent by weight.

One suitable means for dissolving the polymer in the solvent is, by mixing the materials at a temperature above the atmospheric boiling point of the solvent, for example 25 to 120 C. above such boiling point, and at a pressure of 2 to atmospheres for a period of 1 to 5 hours.

Preferably, the polymer solutions, after suitable filtration to remove any undissolved portions, are dry spun. For example, the solutions may be extruded through a spinneret into a conventional type downdraft spinning column containing a circulating inert gas such as nitrogen, noble gases, combustion gases or superheated steam. Conveniently, the spinneret face is at a temperature of from about 100 to 170 C., the top of the column from about 120 to 220 C., the-middle of the column from about 140 to 250 C., and the bottom of the column from about 160 to 320 C. After leaving the spinning column, the continuous filamentary materials are taken up, for example, at a speed within the range of about 50 to 350 meters or more per minute. If the continuous filamentary materials are to be washed while wound on bobbins, the resulting as-spun materials may be subjected to a slight steam drawing treatment at a draw ratio of from about 1.05:1 to 1.5 :1 in order to prevent the fibers from relaxing and falling off the bobbin during the subsequent washing step. Further details with respect'to a method for dryspinning a continuous length of a polybenzimidazole fibrous material are shown in U.S.Pat. No. 3,502,576 to Bohrer et al. which is-assigned to the same assignee as the present invention and is herein incorporated by reference.

Residual spinning solvent is next removed from the continuous length of polybeuzimidazole fibrous material so that the fibrous material contains less than about 1 percent by weight solvent based on the weight of the continuous filamentary material, and preferably so as to obtain an essentially spinning solvent-free fibrous material..(i.e., a fibrous material containing less than about 0.1 percent solvent by weight), Typically, a simple water wash is employed; however, other solvent removal techniques or other wash materials such as acetone, methanol, methylethyl ketone and similar solvent-miscible and volatile organic solvents may be used in place of or in combination with the water. The washing operation may be conducted by collecting the polybenzimidazole fibrous material on perforated rolls or bobbins, immersing the rolls in the liquid wash bath and pressure washing the fibrous material, for example, for about 2 to 48 hours or more. Alternatively, the continuous length of polybenzimidazole fibrous material may be washed on a continuous basis by passing the fibrous material in the direction of its length through one or more liquid wash baths (e.g., for 1 to 10 minutes). A particularly preferred wash technique is disclosed in my commonly assigned U.S. Ser. No. 239,973, filed Mar. 31, 1972. Any solvent removal technique known to those skilled in the art may be selected.

The continuous length of polybenzimidazole fibrous material may next be dried to remove the major proportion of the 'water wash bath and/ or moisture otherwise in association therewith by any convenient technique. For instance, the drying operation for bobbins of yarn may be conducted at a temperature of about 150 to 300 C. for about 2 to hours or more. Alternatively, the continuous length of polybenzimidazole fibrous material may be dried on a continuou basis by passing the fibrous material in the direction of its length through an appropriate drying zone (e.g., suspended in an oven provided at 300 to 400 C. for 1 to 2 minutes). If drying is employed, preferably the drying temperature does not exceed about 250 C. for several hours or 400 C. for more than one minute, as above these limits degradation of the fiber may occur. The polybenzimidazole fibrous mate rial when contacted with the auxiliary heated surface (described hereafter is in association with a minor quantity of moisture (e.g., about 0.5 to 2 percent by weight). It is accordingly unnecessary to remove the final minor quantity of moisture from the fibrous material which has proven to be the most difficult to remove. Also, the rapid absorption of a minor quantity of moisture from the atmosphere following drying can now be effectively tolerated without deleterious results.

The polybenzimidazole fibrous material prior to processing in accordance with the present process preferably possesses a denier per filament of about 1 to 20, and most preferably about 3 to 16 (e.g., 3 to 6 for a multifilament tow and 8 to 16 for a multifilament yarn). Multifilament yarns selected for use in the process preferably contain about 10 to 500 filaments, and most preferably about 10 to 200 filaments. A multifilament tow selected for use in the process preferably contains about 1,000 to 300,000 filaments, or more, and most preferably about 50,000 to 150,000 filaments. When tows containing an extremely large number of filaments are drawn in accordance with the present invention, it is preferred that the tows be supplied to the drawing zone (described hereafter) u-hile in a flattened ribbon-like configuration.

Improved Drawing Procedure The hot drawing step of the present process is accomplished through the application of a longitudinal tension to the continuous length of polybenzimidazole fibrous material while in contact with at least one heated draw surface from which sufiicient heat is imparted to the fibrous material to heat the same to a temperature which exceed the glass transition temperature of the polybenzimidazole.

' The continuous length of polybenzimidazole fibrous material which is in association with about 0.5 to 2 percent moisture by weight immediately prior to contact with the heated draw surface is contacted with at least one auxiliary heated surface in the absence of elongation whereby suflicient heat is imparted to the fibrous material to heat the same to a temperature within the range of about 200 C. up to-below the glass transition temperature of the polybenzimidazole with the simultaneous removal of moisture present in association therewith. The

auxiliary heated surface which contacts the polybenzimidazole fibrous material containing a minor quantity of water may be provided in a variety of configurations. For instance, the auxiliary heated surface may assume the configuration of one or more heated shoes, rollers, pins, or plates.

As is known to those skilled in the art, the glass transition temperature will vary with the specific polybenzimidazole fibrous material selected for drawing and therefore must be taken into consideration when selecting process conditions. Commonly, polybenzimidazole glass transition temperatures fall within the range of about 250 to 450 C. and may be determined by simple experimentation, e.g., while employing differential thermal analysis. The polybenzimidazole fibrous material commonly is heated to a temperature of about 200 to 380 C. while in contact with the auxiliary heated surface, and preferably to a temperature of about 325 to 375 C. When the continuous length of polybenzimidazole fibrous material contains a relatively large number of filaments (e.g., is a large multifilament tow), it is possible to employ a heated auxiliary surface which is provided at a temperature above the glass transition temperature of the polybenzimidazole. Because of the size of the mass of the fibrous material in contact with the same, the residence time concomitantly is adjusted so that insufficient heat is imparted for the fibrous material to reach the glass transition temperature. Fiber temperatures While in contact with the auxiliary heated surface may be determined by techniques known to those skilled in the art, such as by infra-red analysis.

The minimum residence time while in contact with the auxiliary heated surface is dependent upon the single filament denier of the fibrous material, and the number of filaments in the continuous length of fibrous material, as well as the temperature of the auxiliary heated surface. Simple experimentation will enable optimum residence times to be obtained. Typically, however, when filaments of about 1 to 20 denier are present in a yarn of about 10 to 200 filaments, residence times of about 0.2 to 5 seconds, preferably 0.4 to 1.5 seconds, and more preferably 0.5 to 1 second are employed. When processing tows of about 50,000 to 150,000 filaments of 1 to 20 denier per filament, typically the residence times are about 0.2 to seconds, preferably 1 to 8 seconds, and more preferably 3 to 5 seconds. The shorter residence times are associated primarily with smaller denier filaments and smaller yarns and tows. During such contact the continuous length of polybenzimidazole fibrous material is passed over the auxiliary heated surface in the direction of its length while under a longitudinal tension insutficient to elongate the same due to the yielding of the same on the heated draw surface (described hereafter) provided at a higher temperature.

The drawing step of the process is carried out immediately following contact of the continuous length of polybenzimidazole fibrous material with the auxiliary heated surface. The heated draw surface may be conveniently positioned adjacent the auxiliary heated surface and many assume the configuration of one or more heated draw shoes, rollers, pins, plates, etc. In a preferred embodiment of the process the heated draw surface is maintained at a temperature sufiicient to heat the polybenzimidazole fibrous material in contact therewith to a temperature within the range of about 390 to 500 C., and most preferably to a temperature within the range of about 425 to 475 C. When the continuous length of polybenzimidazole fibrous material contains a relatively large number of filaments (e.g., in a large multi-filament tow) it is possible to employ a heated draw surface which is provided at a temperature in excess of that imparted to the fibrous material in contact therewith. Because of the size of the mass in contact with the same, the residence time concomitantly is adjusted so that the desired fiber temperature is achieved.

While in contact with at least one heated draw surface,

the fibrous material preferably is drawn at a draw ratio of about 1.5 :1 to 4.5 :1, and most preferably at a draw ratio of about 2:1 to 3.5 :l. The term draw ratio, as is well known, is a measure of the degree of stretching during the drawing or orientation of the fibrous material expressed as the ratio of the cross-sectional area of the undrawn material to that of the drawn material. While any of the several known ways for measuring or determining draw ratio may be employed, typically the draw ratio is found by taking the ratio of the surface speed of a takeup roll following contact with the heated draw surface to the surface speed of a feed or supply roll prior to contact with the heated draw surface.

The drawing speed utilized in the present process preferably is at least 10 meters per minute. The drawing speed is defined as the rate at which the continuous length of fibrous material is supplied to the heated draw surface while under a longitudinal tension. The drawing speed utilized in the present process is preferably about 10 to 50 meters per minute when drawing a multifilament tow, and preferably about 20 to meters per minute when processing a multifilament yarn.

The minimum hot drawing residence time, i.e., the time during which the continuous length of fibrous material is in contact with the heated draw surface while under a longitudinal tension, is dependent upon the single filament denier of the fibrous material, and the number of filaments in the continuos length of fibrous material, as

well as the temperature of the heated draw surface.'

Simple experimentation will enable the optimum residence times to be obtained. Typically, however, when filaments of about 1 to 20 denier are present in a yarn of about 10 to 200 filaments, residence times of about 0.05 to 3 seconds, preferably 0.1 to 1.5 seconds, and more preferably 0.2 to 1 second are employed. When processing tows of about 50,000 to 150,000 filaments of 1 to 20 denier per filament, typically the residence times are about 0.05 to 7.5 seconds, preferably 0.1 to 5 seconds, and more preferably 0.2 to 3 seconds. The shorter residence times are associated primarily with smaller denier filaments and with smaller yarns and tows. During such contact the continuous length of polybenzimidazole fibrous material is passed over the heated draw surface in the direction of its length while under a longitudinal tension sufficient to elongate the same to the desired degree.

The nature of the gaseous atmospheres provided adjacent the auxiliary heated surface and the heated draw surface are not critical to the operation of the present process. A gaseous atmosphere is preferably selected, however, which is substantially unreactive with the fibrous material passing through the same. For instance, air may conveniently serve as the gaseous atmosphere. Inert gaseous atmospheres such as nitrogen, argon, or helium, also may be selected.

The brief treatment of the continuous length of polybenzimidazole fibrous material while in contact with the auxiliary heated surface has surprisingly been found capable of eliminating the heretofore difiicultly removable minor quantity of moisture in association therewith in a substantially non-deleterious manner, and to improve the stability of the overall hot drawing process. Additionally, the tensile properties of the resulting drawn polybenzimidazole fibrous material are superior to those achieved if contact with the auxiliary heated surface were omitted or if the auxiliary heated surface raised the fiber'to the glass transition temperature of the samerOpaque streaking within the drawn fibrous product, which is commonly associated with areas of low elongation, is minimized or completely eliminated. 1

The following Examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific details set forthinthe Examples.

EXAMPLES I,IV

A continuous filament polybenzimidazole yarn formed from poly 2,2 (m-phenylene)-5,5'-bibenzimidazole was selected to exemplify the process of the present invention.

For comparative purposes Examples I-IV were repeated with the exception that the pair of heated shoes (i.e., the auxiliary heated surface and the heated draw surface) were provided at the same temperature which equaled or exceeded the glass transition temperature of the fibrous 1 The polymgr formgd mm an as'spun yam m the material. More specifically, the pair of shoes was provided manner described 1n the Example I of US. Pat. No. f l a o f at each 0 the fol owing temperatures. 400 C., 410 C., 3,502,756 to Bohrer et al. More specifically, a dope o C C c C C 470 C V and N N-dlmethylacetamide containing 23 percent by weight a 1 t uded throu h a whole into 480 C. In all lnstances the process instability was so 0 d 6 P i g min en as drying 10 great that a drawn product could not be collected since a W g f am g g ratio of fiber breakage took place as moisure was evolved in a inmosp an was 5 re disruptive manner during sudden heat-up while in contact in a steam atmosphere to form a 600/50 yarn (50 filawith the auxiliary heated surface ments making up a yarn having an overall dry denier of 600). The yarn was washed with water while wound upon A E H a bobbin until the residual solvent content was less than EX MPL S V v I P by The gi f i to a? A continuous filament polybenzimidazole tow formed Y all but a mmor quanmy o 1 cu y e from poly-2,2 (m-phenylene)-5,5-bibenzimidazole was molstul'e from j Same by Placemm?t of the yam a selected to further exemplify the process of the present heated oven While present upon a bobbin. invention The Y was next unwound from the bobbin and was The tow consisted of 1000 filaments and had a total Passed While an open atmosphere e apalr 9 heated denier of 2700. The tow was washed with water and had Shoes P f m endEto i relgtlgnshlp g wider a residual solvent content (i.e., a N,N-dimethylacetamide gfltig ggl linsalvteflsigle. firasi sfiogebeiarllg 1 6F31 5 3112 5213 gontgntt) of less thafi 03.01 percent by weight. Tlt etgiv vtias H ne 0 remove a ut a minor quantity 0 i cu ty 1 1:3; h g gsi (igfs sur al f T lig gggn ii gi d 5:1 removable moisture from the same by being heated in 1 an oven. transltlon temperaturfi about 2 g h The tow was next passed at various speeds while under 23235.?.2322:.aszraarr wttr.farm; a 2; an a pan 0 eate s oes p ace in an en to en re ationiliary heated surface at rateof 45 meters P mmute ship. Each shoe had a contact surface of 13 inches, with a W taken P fonqwlng wlthdrawal from h heated the first shoe being termed the auxiliary heated surdraw s rfac at a rate of 100 meters P mlnute- N face and the second shoe being termed the heated draw elongation took Place While the Y inPontact Wlth surface. The tow exhibited a glass transition temperathe auxiliary heated surface fora residence time of about m f about 425 C, d contained about 15 percent 0.4 second where the yarn was heated to substantially moisture by weight when fed to the auxiliary heated surthe same temperature as the auxiliary heatedsurface. The face. No elongation took place while the tow was in conyarn was (drawn at a dgazrattig 20f 2-2illwhtlklle ti; cgnttac tact with the auxiliarybheatesd surgace, agd1 was Illieated up for a resi ence time o a on. secon W1 e ea c to a temperature 0 a out to 1 C. e ow t at of the draw surface which exceeded the glass transition temauxiliary heated surface. While the tow was in contact perature of the polybenzimidazole.-While in contact with with the heated draw surface it was heated up to a temthe heated draw surface, the yarn was heated to substanperature of about 5 to 10 C. below that of the heated tially the same temperature as the heated draw surface. draw surface and its glass transition temperature was Summarized below in Table A are the process paramexceeded. The tow was drawn at a draw ratio of 2:1 while eters utilized and the results achieved. in contact with the heated draw surface.

' TABLE A Temperature C.) ot- Properties of product Auxiliary Heated Denier Tenacity, Elonga- Example heated draw Processing per grams/ tion, number surface surfaceu stability filament denier percent TE1/2 Appearance 390 425 Poor. 5.2 4.52 8.8 13.4 Opaque. 365 440 Better 5.1 5. 41 18.0 22.8 Some streaks. 365 452 Good 5.4 5.01 23.3 24.0 Occasional streaks. 355 475 Very good--. 5.2 5.47 24.2 27.0 No streaks.

*TE "=Index of fiber organization wherein T is tenacity at break in grams per denier and E is elongation in percent extension from original length at break in tensile test. An explanation of this test and its significance is given in the Textile Research Journal 36, No. 7, pp. 593-602, July 1966.

In Examples I-1V moisture in association with the polybenzimidazole yarn was removed in a substantially non- Summarized below in Table B are the process parameters utilized and the results achieved.

deleterious manner while the yarn was in contact with the auxiliary heated surface. Optimum results were achieved as the temperature of the auxiliary heated surface was decreased and the temperature of the heated draw surface was increased.

In examples V-VIII moisture in association with the polybenzimidazole tow was removed in a substantially non-deleterious manner while the tow was in contact with the auxiliary heated surface. Satisfactory drawing stability 1 1 was achieved, and the product in each instance was free from streaking.

For comparative purposes Examples V-VIII were repeated with the exception that the auxiliary heated surface was heated to the same temperature as the heated draw surface. Satisfactory drawing stability was unattainable since fiber breakage took place as moisture was evolved in a disruptive manner during sudden heat-up while in contact with the auxiliary heated surface.

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto,

I claim:

1. In the process for producing drawn polybenzimidazole continuous filamentary materials wherein polybenzimidazole is wet or dry solution spun into a continuous filamentary material, the resulting as-spun filamentary material is washed to remove residual spinning solvent, the washed filamentary material is dried in order to remove moisture, and the dried polybenzimidazole continuous filamentary material is thereafter drawn, the improvement which comprises:

(a) introducing the washed, moisture-containing polybenzimidazole fibrous material into a drying zone in order to reduce the moisture content thereof to about 0.5 to 2 percent by weight;

(b) continuously passing the dried material resulting from step (a) into contact with at lease one heated surface whereby suificient heat is imparted to said fibrous material to heat said material to a temperature within the range of about 200 C. up to below the glass transition temperature of said polybenzimidazole in order to remove the remaining moisture present in association therewith, said moisture removal being conducted in the absence of any elongation of said fibrous material; and

(c) immeditely thereafter drawing the resulting moisture-free material obtained from step (b) at a draw ratio of about 1.5 :1 to 4.5 :1 while in contact with at least one heated draw surface from which sufficient heat is imparted to said fibrous material to heat the same to a temperature which exceeds the glass transition temperature of said polybenzimidazole to yield the desired drawn continuous length of polybenzimidazole fibrous material.

2. The process of claim 1 wherein the fibrous material is drawn while in contact with at least one heated draw surface maintained at a temperature suflicientito heat the fibrous material in contact therewith to a temperature within the range of about 390 to 500 C.

3. The process of claim 2 wherein the fibrous material is heated in step (b) to a temperature below the 12 glass transition temperature of said polybenzimidazole but within the range of about200 to 380 C.

4. The process of claim 1 wherein the fibrous material is drawn at a draw ratio of about 2:1 to 35:1 while in contact with at least one heated drawsurface maintained at a temperature suflicient to heat thefibrous material in contact therewith to a temperature Within'the range of about 425 to 475 C., and wherein the fibrous material is heated in step (b) to a temperature below the glass tran sition temperature of said polybenzimidazole but within the range of about 325 to 375 C.

5. The process of claim 3 wherein said polybenzimidazole fibrous material consists essentially ofrecurring units of the formula: I v

wherein R is a tetravalent aromatic nucleus, with the nitrogen atoms forming the benzimidazole rings paired upon adjacent carbon atoms of said aromatic nucleus,

and R is selected from the group consisting of (1) an aromatic ring, (2) an alkylene group having from 4 to 8 carbon atoms, and (3) a heterocyclic ring selected from the group consisting of (a) pyridine, (b) pyrazine, (c)

furan, (d) quinoline, (e) thiophene, and (f) pyran.

6. The process of claim 4 wherein said polybenzimidazole fibrous material is poly-2,2-(m-phenylene)-5,5'-bibenzimidazole.

7. The process of claim 5 wherein said continuous length of polybenzimidazole fibrous material is a multifilament yarn.

8. The process of claim 5 wherein said continuous length of polybenzimidazole fibrous material is a multifilament tow.

References Cited UNITED STATES PATENTS 

